Add exception tracing to django middleware

[tonybaloney]

Adds additional functionality to the Django middleware to capture exceptions and add the error message, type, and traceback to the trace span attributes.

Will attempt to use both Python 2 and 3 tracebacks

I'm new to this project, so please let me know if there's anything that needs changing

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[tonybaloney]

CLA signed

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CLAs look good, thanks!

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[lzchen]

Thanks for the addition!

Could you fix the build errors (I believe it's because we changed black_list to exclude_list #977) and add a CHANGELOG entry as well?

[tonybaloney]

Thanks for the addition!

Could you fix the build errors (I believe it's because we changed black_list to exclude_list #977) and add a CHANGELOG entry as well?

@lzchen I can't see why the py37 build is failing? The others are fixed

[lzchen]

@tonybaloney
The lint build is failing:
contrib/opencensus-ext-django/tests/test_django_middleware.py:351:80: E501 line too long (84 > 79 characters)

[lzchen]

@tonybaloney
Are you still working on this? There are some changes that I would like to get released and if we could get this change in too that'd be great.

[tonybaloney]

@lzchen nope, its finished if you're happy with it?

[lzchen]

@tonybaloney
I've made some comments above that need to be addressed.

[tonybaloney]

@lzchen requested amendments made

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